Apparatus and method for securing communication information in CDMA communication system

ABSTRACT

An apparatus and a method for securing a communication information in a CDMA communication system are disclosed. The method of the invention comprises the steps of encoding a input analog signal as an information bit having a predetermined size and generating a vocoder packet information bit, and encrypting said encoded vocoder packet information bit using a block cipher and a security key, and adding a frame quality indicator and the encoder tail bits to the encrypted vocoder packet information bit and configuring it as a CDMA frame, and transmitting the CDMA frame which passes a convolutional encoder, interleaver, and modulator in sequence, to a base station through an assigned frequency band. 
     Further, The method of the invention comprises the steps of receiving a signal from a base station, and reproducing it as a CDMA frame, and extracting an encrypted vocoder packet information bit from the reproduced CDMA frame, and decrypting the encrypted vocoder packet information bit by a block cipher and a security key, and decoding the decrypted vocoder packet information bit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a CDMA communication system, and moreparticularly to an apparatus and a method for securing a communicationinformation in a CDMA communication system, thereby enabling the systemto communicate privately with no overhearing.

2. Description of the Related Art

In a conventional CDMA communication system according to the IS-95standard, a voice is encoded as an information bit by a vocoder (a voiceencoder/decoder) and modulated in a reverse traffic channel. Themodulated signal is transmitted to a base station. The modulated signalfrom the base station is reproduced as the original information bit in aforward traffic channel and it is decoded as an original voice by avocoder. By the above process, it is possible to communicate between theremote mobiles.

However, the conventional CDMA system has the disadvantage that theinformation bit passing through only the vocoder can be detected at thebase station or at a switching system, and the detected information bitcan be easily decoded as the original voice. Thus, the conventional CDMAsystem has weakness for an overhearing.

U.S. Pat. No. 5,727,064 discloses the technique for solving thedisadvantage, wherein a scrambler is added to a longcode generator. But,the technique can not be applied to the conventional CDMA system withoutany change because it requires the modification of IS-95 CDMAfacilities, and it has a limit to communicate between the different usergroups using the different security keys.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus and amethod for securing a communication information in a CDMA communicationsystem, which enables the system to communicate privately with nooverhearing while the modification of the conventional base station andthe switching system according to IS-95 standard is not required.

The foregoing object is accomplished in the present invention by anapparatus for securing communication information in CDMA communicationsystem comprising a vocoder encoding the input analog signal as aninformation bit having a predetermined size and generating a vocoderpacket information bit, and an encryptor encrypting the vocoder packetinformation bit from said vocoder, and a CDMA framer adding a framequality indicator and the encoder tail bits to the encrypted vocoderpacket information bit from said encryptor to configure as a CDMA frame,and a CDMA frame transmitter transmitting the CDMA frame which passes aconvolutional encoder, interleaver, and modulator in sequence, to abasestation through an assigned frequency band, and a CDMA frame receiverreceiving a signal from the base station and reproducing the CDMA frame,and a CDMA deframer extracting the encrypted vocoder packet informationbit from the CDMA frame reproduced by said CDMA frame receiver, and adecryptor decrypting the encrypted vocoder packet information bitextracted by said CDMA deframer, and a vocoder decoding the decryptedvocoder packet information bit from said decryptor as an analog signal,wherein said encryptor encrypts the vocoder packet information bit usinga block cipher and a security key, said decryptor decrypts the encryptedvocoder packet information bit using said block cipher and a securitykey shared with the other mobile.

In addition, the foregoing object is accomplished in the presentinvention by providing a method for securing communication informationin CDMA communication system comprising the steps of encoding a inputanalog signal as an information bit having a predetermined size andgenerating a vocoder packet information bit, and encrypting said encodedvocoder packet information bit using a block cipher and a security key,and adding a frame quality indicator and the encoder tail bits to theencrypted vocoder packet information bit and configuring it as a CDMAframe, and transmitting the CDMA frame which passes a convolutionalencoder, interleaver, and modulator in sequence, to a base stationthrough an assigned frequency band.

The foregoing object is also accomplished in the present invention byproviding a method for securing communication information in CDMAcommunication system comprising the steps of receiving a signal from abase station, and reproducing it as a CDMA frame, extracting anencrypted vocoder packet information bit from the reproduced CDMA frame,and decrypting the encrypted vocoder packet information bit by a blockcipher and a security key, and decoding the decrypted vocoder packetinformation bit.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be moreclearly understood from the following description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a block diagram showing the traffic channel according to thepresent invention;

FIG. 2 shows a DES algorithm applicable to the present invention;

FIG. 3 shows an illustrated single iteration in the DES algorithm ofFIG. 2;

FIG. 4 shows a flow diagram of encryption/transmission process accordingto a preferred embodiment of the present invention;

FIG. 5 is a flow diagram showing reception/decryption process accordingto a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Reverse/Forward Traffic Channel

FIG. 1 is a block diagram showing the traffic channel according to thepresent. In FIG. 1, a dotted block shows an essential part of theinvention.

Referring to FIG. 1, a reverse traffic channel will be described in thefollowing.

A voice from a microphone is encoded as a vocoder packet information bit141 by a vocoder 100, and transmitted to an encryptor 140. Using a blockcipher 170 such as DES and triple DES, the encryptor 140 encrypts all orthe part of the vocoder packet information bits 141 from the vocoder 100and generates an encrypted vocoder packet information bit 142 and sendsit to a CDMA framer 110. In the encryption process, a security key 160is used, which is stored in the encryptor 140 or exchanged in a safemanner.

A CDMA framer 110 performs the process for making a CDMA frame by addinga frame quality indicator and the encoder tail bits to the encryptedvocoder packet information bit 142.

Now, Referring to FIG. 1, a forward traffic channel will be described.

Using a block cipher 170 identical to that in the reverse channel, adecryptor 150 decrypts all or the part of the encrypted vocoder packetinformation bits 152 from a CDMA deframer 120 to reproduce the originalvocoder packet information bit 151 and sends them to a vocoder 130. Inthe decryption process, a security key 160 is also used, which is storedin the decryptor 150 or exchanged in a safe manner. Here, the CDMAdeframer 120 performs the reverse process as that of the CDMA framer110.

2. Encryption/Decryption

The encryption of the vocoder packet information bit 141 and thedecryption of the encrypted vocoder packet information bit 152 areperformed in bite by the block cipher 170, and thus only the informationbits corresponding to a multiple of 8 are encrypted or decrypted.

For Example, in IS-95A 9600bps frame, only 21 bytes areencrypted/decrypted since the number of the information bits are 172bits (=8×21+4). At the information bits of 172 bits (1) if it's a fullrate, all 171 bits (vocoder output) excepting the first 1 bit Format bit(0) are encrypted/decrypted, and (2) if it isn't a full rate (forexample, ½ rate, 80 bits), the bits, which corresponds to output of thevocoder, of the remaining 168 bits excepting the first 4 bits Formatbits (for example ½ rate 1000), that is, only a part of the ½ bits areencrypted/decrypted. However, when the amount of the calculations aretoo much to be encrypted/decrypted in a given time, only the part ofthem (for example, 8 bytes) are encrypted/decrypted. In addition, whenthe output of the vocoder 100 is a mute (⅛ rate), (which is different insize according to vocoder, and is usually discriminated throuah formatbits) the encryption/decryption process may be omitted in order toprevent a repeated transmission of the same pattern or in or to preventa noise. Using a software or a hardware, the encryptor 140 and decryptor150 performs the encryption/decryption process every 20 ms after thevocoder completes the encoding.

For the encryption/decryption, assume the case using the DES (DataEncryption Standard) selected as a FIPS PUB 46 (Federal InformationProcessing Standard 46) by the NIST (National Institute of Standards andTechnology).

FIG. 2 shows a DES algorithm applicable to the present invention andFIG. 3 shows an illustrated single iteration in the DES algorithm ofFIG. 2 using a 64 bits text and 56 bits security key. In this case, forexample, assume that only the first 8 bytes of the 172 bits areencrypted/decrypted in a full rate. Then, the 64 bits plain text (forexample, 01 23 45 67 89 ab cd e7) of FIG. 2 expresses the first 8 byteswhich corresponds to pure output of the vocoder of the 172 vocoderpacket information bits transmitted from the vocoder 100, and 56 bitskey (for example, 01, 23, 45, 67 89 ab de) of FIG. 2 indicates asecurity key 160 which a sending and a receiving mobile are sharing. The172 encrypted vocoder packet information bits are obtained by adding theremaining unencrypted 108 bits to the 64 bits cipher text (for example,c9 57 44 25 6a 5e d3 1d) from the encryptor 140 in front and back of theformal bits and the remaining vocoder output.

A decryption process, which is a reverse process of the encryption, willbe described. The original 64 bits plain text (for example, 01 23 45 6789 ab cd; FIG. 4) is obtained by decrypting the 8 encrypted bytes (i.e.64 bits cipher text in FIG. 2 (for example, c9 57 44 25 6a 5e d3 1d))among the 172 encrypted vocoder packet information bits from the CDMAdeframer 120 and 56 bits key in FIG. 2 (for example, 01 23 45 67 89 abcd), in a reverse order as in the encryption process. The 172 vocoderpacket information bits generated by adding a remaining 108 bits to the64 bits plain text are transmitted to the vocoder 130 and decodedtherein to reproduce the original voice.

The block cipher such as DES and triple DES is used for theencryption/decryption process.

3. Security Mode On/OFF

For a communication with the mobiles to which the above security methodcan not be applied, a sending mobile is set to the security mode On orOff if a predefined key is entered at the beginning of thecommunication, and a receiving mobile is also set to the security modeOn or Off identically to the sending mobile. This enables the remotemobiles to communicate privately without any overhearing. Such securitymode ON/OFF with the predefined key may be performed before a callsetup, or may be performed during the communication after a call setup.

The security mode ON is performed as the following. An ON key requestingthe security mode is entered in a reverse traffic channel. Then, a firstpattern (distinguished from the vocoded voice) corresponding to thesecurity mode ON is transmitted to the other mobile with the informationbit. The other mobile receives and checks the first pattern to set thesecurity mode ON.

The security mode OFF is performed as the following. An OFF key isentered during the communication and a second pattern (distinguishedfrom the vocoded voice) corresponding to the security mode OFF istransmitted to the other mobile with the information bit. The othermobile receives and checks the second pattern to set the security modeOFF and returns back to the normal mode. Alternatively, the securitymode may be automatically set to OFF when the communication iscompleted. With this method, it is possible to repeat the security modeON/OFF during the communication.

In a weak wave area, an error may be occurred in the patternstransmitted for the security mode ON/OFF and thus it may be happenedthat the receiving mobile can not be set to the security mode ON/OFF.Then, it is impossible to communicate between the remote mobiles becausethe sending mobile is set to the security mode On and the receivingmobile to the security mode OFF. This can be settled with the followingsolutions; 1) sending the security mode ON/OFF signal repeatedly toreduce the occurrence of the error, or 2) informing the receipt of thesecurity mode On/OFF signal by sending a ECHO signal from the receivingmobile to the sending mobile when the sending mobile transmits thesecurity mode On/OFF signal to the receiving mobile and communicatingeach other after the sending mobile detects the ECHO signal from thereceiving mobile and sets the security mode to ON/OFF.

The ON key and the OFF key used to set the security mode ON/OFF areselected among the keys which does not affect the communication.Further, in the case that the security mode is set to ON, the securitykey in the block cipher may be transmitted with the ON key or a schemefor finding out the security key may be transmitted with the ON key,which will be described in the following.

4. Mute Handling

Generally, a mute denotes a condition that no voice is detected for 20ms. In the case of the mute, the encryption process in the encryptor 140and the decryption process in the decryptor 150 may be bypassed by thesignals MUTE_(—)Tx 143 or MUTE_(—)Rx 153, and further may be bypassed bya signal Security_(—)Mode 180 in accordance with the security mode.

TABLE 1 Operation of the encryptor in accordance with the security modeand the mute Security Mode Security_(—)Mode Security_(—)Mode MUTE_(—)RxON OFF MUTE_(—)Tx ON bypass bypass MUTE_(—)Tx OFF encryption bypass

TABLE 2 Operation of the decryptor in accordance with the security modeand the mute Security Mode Security_(—)Mode Security_(—)Mode MUTE_(—)RxON OFF MUTE_(—)Rx ON bypass bypass MUTE_(—)Rx OFF decryption bypass

Tables 1 and 2 shows the operation of the encryptor and the decryptor inaccordance with the security mode and the mute, respectively. ReferringTables 1 and 2, the encryption and the decryption are all bypassed whenthe signal Security_(—)Mode 180 is OFF. In addition, altough the signalSecurity_(—)Mode 180 is ON, the encryption and the decryption are alsobypassed when the signals MUTE_(—)Tx 143 or MUTE_(—)Rx 153 is ON.

The signals MUTE_(—)Tx 143 and the MUTE_(—)Rx 153, which indicatewhether the present vocoder packet contains the informationcorresponding to the mute, can be expressed by a flag in a softwareimplementation and can be handled as an additional signal in a hardwareimplementation. The signal Security_(—)Mode 180 indicates whether thesecurity mode is ON or OFF.

In a fixed rate, a separate process is not required when the mutehappens. However, in a variable rate when the mute happens, it ispossible to easily cheek the mute status of the frame through format itsof the vocoder packet without any special process. In this case, theencryption and decryption is not performed. Accordingly, a new logic isnot needed for the implementation of the present invention.

5. Sharing Security Key

The block ciphers for the encryptor 140 and the decryptor 150 use thesame algorithm and the same security key. For this reason, there isrequired a scheme for sharing the security key between a transmitter anda receiver remotely separated from each other.

A first scheme is to send a security key from the transmitter to thereceiver when the security mode is set to ON. For example, when thesecurity mode is set to ON, the 172 information bits in a IS-95 9600 bpsframe are transmitted as a specific pattern (for example, 5555 . . . 555in Hexadecimal) denoting a Security_(—)Mode ON, and then the 128 bits ina next frame (if the security key is 128 bits) is defined as a securitykey, or a security key encrypted by a master key (generally called as asession key and used as a disposable key) is transmitted. When themaster key is used, the mobiles in communication share the same masterkey, which is stored at an authorized organization.

A second scheme is to specify one of the keys stored in a transmitterand a receiver in the same manner (for example, 100 keys of 128 bits)when the security mode is set to ON. In other words, among the 172information bits in a IS-95 9600 bps frame, for example 164 bits aretransmitted as a specific pattern (such as 5555 . . . 5 in hexadecimal)denoting the security mode ON and the remaining 8 bits are used as anindex of the 256 security keys stored in secrecy. At this time, thestored security keys may be configured as the security keys which themobile manufacturer provides and the security keys which a subscriberenters by himself.

A third scheme is that the two subscribers exchange only the securitykey via a separate call. The key to be exchanged can be obtained by thesubscribers' entrance or by a random number generator in the mobile. Thesecurity key exchanged according to the third scheme is specified andused by the second scheme when the security mode is set to ON.

6. Encryption/Transmission Process

FIG. 4 shows a flow diagram of the encryption/transmission processaccording to a preferred embodiment of the present invention. ReferringFIG. 4, an encryption process will be described in detail.

First, when the power of the transmitter is ON, the initialized processin the transmitter is performed (step 400). Consequently, when asubscriber enters a key to request a call (step 405) , the call is setup(step 410). At this time, if the entered key is the security mode ON key(step 415 and 420), then the security mode of the transmitter is set toON (step 425) and the specific pattern indicating the Security_(—)ModeON and an index of the security key selected among the multiple securitykeys in the transmitter are added to the information bit in the trafficchannel (step 430).

If the entered key is the security mode OFF key (step 415 and 420), thenthe security mode of the transmitter is set to OFF (step 435) and thespecific pattern indicating the Security_(—)Mode OFF is added to theinformation bit in the traffic channel (step 440).

If the security mode key is not entered in the step 415, then an inputsignal is encoded as an information bit and the vocoder packetinformation bit is generated by a vocoder (step 445). If the securitymode is on (step 450) and the vocoder packet information bit is not amute (step 445), then the vocoder packet information bit is encryptedusing a block cipher and a security key (step 460). Otherwise, the steps455 and 460 are bypassed. In a CDMA framer, the frame quality indicatorand the encoder tail bits are added to such encrypted or unencryptedvocoder packet information bit to configure a CDMA frame (step 465), andthen the CDMA frame passes a convolutional encoder, interleaver, andmodulator in sequence, and finally transmitted to a base station throughan assigned frequency band (step 470). In a step 475, if the call ismaintained, then the routine is returned to the step 415. If the call iscompleted, then the security mode is set to a normal mode (step 480) andthe routine is returned to the step 405.

Each functions in relation to the encryption process of the inventionmay be implemented by a software or a hardware such as ASIC (ApplicationSpecific Integrated Circuit)

7. Reception/Decryption Process

FIG. 5 shows the flow diagram of the reception/decryption processaccording to a preferred embodiment of the present invention, at thereceiver corresponding to the transmitter in FIG. 4. Referring FIG. 5,the reception/decryption process will be described in detail.

First, when the power of the receiver is ON, the initialized process inthe receiver is performed (step 500). Consequently, when a ring and acall are received (step 505), the call is setup between the transmitterand the receiver (step 510). Then, the receiver receives a modulatedsignal from the base station and reproduces the CDMA frame from thesignal (step 515). The CDMA deframer extracts an encrypted vocoderpacket information bit from the reproduced CDMA frame (step 520). Thedecryptor determines if a security mode pattern is included in theencrypted vocoder packet information bit (step 525). If the securitymode pattern indicates the Security_(—)Mode ON (step 530), then thedecryptor checks an index included in the information bit and determinesa security key corresponding to the index among the multiple securitykeys in the receiver (step 535), and the security mode of the receiveris set to ON (step 540). In the step 530, if the security mode patternin the information bit indicates the Security_(—)Mode OFF, then thesecurity mode of the receiver is set to OFF (step 545).

In the case that the security mode pattern is not included in thevocoder packet information bit in a step 525, if the security mode ofthe receiver is ON (step 550) and the encrypted vocoder packetinformation bit is not a mute (step 555), then the decryptor decryptsthe encrypted vocoder packet information bit using a block cipher andthe security key (step 560). Otherwise, the steps 555 and 560 arebypassed. Such decrypted or bypassed vocoder packet information bit isdecoded by the vocoder (step 565).

If the call is maintained in a step 570, then the routine is returned tothe step 515. If the call is completed in the step 570, then thesecurity mode is set to a normal mode (step 575) and the routine isreturned to the step 505.

Each function in relation to the decryption process of the invention maybe also implemented by a software or a hardware such as ASIC(Application Specific Integrated Circuit).

As described above, the present invention performs the encryptionprocess between the vocoder and CDMA framer in the reverse trafficchannel through the block cipher and performs the decryption processbetween the vocoder and CDMA deframer in the forward traffic channelthrough the block cipher.

Accordingly, since the information bit is transmitted in an encryptedform except for the receiver and transmitter, it is impossible todecrypt the encrypted information bit as the original vocoder packetwithout exactly knowing the algorithm and the security key, even if theencoded information bit is detected by an advanced technique.

Further, the invention has the advantage that, without the modificationof the conventional base station and the switching system according toIS-95 standard, it enables the CDMA system to communicate privately withno overhearing.

The invention also provides the simple and safe method for sharing thesecurity key, to thereby allow the communication between the differentuser groups using the different security key. Thus, it is possible tobuild up the security network readily among the small user groups.

The present invention has been described in terms of preferredembodiments. However, it should be understood that the present inventionis not limited in its application to the specific embodiments. Thoseskilled in the art will recognize that various modifications andvariations may be made without departing from the spirit and scope ofthis invention, as defined in the following claims.

1. A terminal for CDMA mobile communication system of IS-95 standardhaving data protection function, comprising: a vocoder encoding theinput analog signal as an information bit having a predetermined sizeand generating a vocoder packet; an encryptor encrypting the vocoderpacket from said vocoder and generating the encrypted vocoder packet; avocoder framer adding a format bit/format bits indicating the vocoderrate and, as a result, generating the encrypted vocoder frame; a CDMAframer adding a frame quality indicator and the encoder tail bits to theencrypted vocoder frame from said vocoder framer to configure as a CDMAframe; a CDMA frame transmitter transmitting a CDMA frame which passes aconvolutional encoder, interleaver, and modulator in sequence, to a basestation through an assigned frequency band; a CDMA frame receiverreceiving a signal from the base station and reproducing the CDMA frame;a CDMA deframer extracting the encrypted vocoder frame from the CDMAframe reproduced by said CDMA frame receiver; a vocoder deframerdeleting a format bit/format bits indicating the vocoder rate from theencrypted vocoder frame and, as a result, extracting the encryptedvocoder packet; a decryptor decrypting the encrypted vocoder packetextracted by said vocoder deframer; and a vocoder decoding the decryptedvocoder packet from said decryptor as an analog signal.
 2. The terminalfor CDMA mobile communication system having data protection functionaccording to claim 1, wherein said encryptor encrypts using a blockcipher such as DES, 3DES or a stream cipher, and a secret key, saiddecryptor decrypts using said block cipher or said stream cipher and thesecret key shared by the other terminal.
 3. The terminal for CDMA mobilecommunication system having data protection function according to claim1, wherein said encryptor performs or bypasses the encryption by asecurity mode that a user set to ON or OFF.
 4. The terminal for CDMAmobile communication system having data protection function according toclaim 3, wherein said encryptor generates a predetermined patterncorresponding to ON or OFF of the security mode and adds the generatedpattern to the format bit/bits of the traffic channel, and transmits itto the other terminal, wherein said decryptor performs or bypasses thedecryption by setting ON or OFF of said security mode by said receivedpattern.
 5. The terminal for CDMA mobile communication system havingdata protection function according to claim 3, wherein said encryptoradds the encrypted secret key to the format bit/bits and thepredetermined security mode pattern of the traffic channel when thesecurity mode is set to ON, wherein said decryptor recovers the secretkey included in the vocoder frame of the traffic channel, and decryptsthe encrypted vocoder packet using said recovered secret key.
 6. Theterminal for CDMA mobile communication system having data protectionfunction according to claim 1, wherein said encryptor encrypts all or athe of the vocoder packet, wherein said decryptor decrypts all or a theof the encrypted vocoder packet.
 7. A method for encrypting andtransmitting information bit in CDMA mobile communication system ofIS-95 standard, comprising the steps of: (a) encoding an input signal asan information bit having a predetermined size and generating a vocoderpacket; (b) encrypting said encoded vocoder packet; (c) adding a formatbit/format bits indicating the vocoder rate, resulting the encryptedvocoder frame, and then adding a frame quality indicator and the encodertail bits to the encrypted vocoder frame and configuring it as a CDMAframe; and (d) transmitting the CDMA frame which passes a convolutionalencoder, interleaver, and modulator in sequence, to a base stationthrough an assigned frequency band.
 8. A method for encrypting andtransmitting information bit in CDMA mobile communication systemaccording to claim 7, wherein said step (b) encrypts using a blockcipher such as DES, 3DES or a system cipher, and a secret key.
 9. Amethod for encrypting and transmitting information bit in CDMA mobilecommunication system according to claim 8, wherein said step (b)performs or bypasses the encryption by a security mode that a user set.10. A method for encrypting and transmitting information bit in CDMAmobile communication system according to claim 9, wherein said step (b)generates a predetermined pattern corresponding to ON or OFF of thesecurity mode and adds the pattern to the information bit of the trafficchannel.
 11. A method for encrypting and transmitting information bit inCDMA mobile communication system according to claim 10, wherein saidstep (b) adds said secret key to the information bit of the trafficchannel when the security mode is set to ON.
 12. The method forencrypting and transmitting information bit in CDMA mobile communicationsystem according to claim 7, wherein said step (b) encrypts only a partof the vocoder packet.
 13. A method for receiving and decryptinginformation bit encrypted by terminal for CDMA mobile communicationsystem of IS-95 standard, comprising the steps of: (a) receiving asignal from a base station, and reproducing it as a CDMA frame; (b)extracting an encrypted vocoder frame from the reproduced CDMA frame;(c) extracting the encrypted vocoder packet from the extracted vocoderframe; (d) decrypted the encrypted vocoder packet; and (e) decoding thedecrypted vocoder packet.
 14. The method for receiving and decryptinginformation bit encrypted by terminal for CDMA mobile communicationsystem according to claim 13, wherein said step (b) decrypts using ablock cipher such as DES, 3DES or a system cipher, and a secret key. 15.The method for receiving and decrypting information bit encrypted byterminal for CDMA mobile communication system according to claim 14,wherein said step (d) set a security mode to ON or OFF according to thepredetermined pattern included in the received information bit of thetraffic channel, and determines according to the security mode whetherthe decryption process is to be performed or not.
 16. The method forreceiving and decrypting information bit encrypted by terminal for CDMAmobile communication system according to claim 15, wherein said secretkey is that included in the information bits of the traffic channelreceived when the security mode is set to ON.
 17. The method forreceiving and decrypted information bit encrypted by terminal for CDMAmobile communication system according to claim 13, wherein said step (d)decrypts only a part of the encrypted information bit from the encryptedvocoded packet.